Wow! This topic has come back to life again (or is that again and again and again?)! Since it's been so long since I popped into this thread, it might help if I comment again for the benefit of people who are newer to the forums.

Sorry to disappoint everyone, but Heather is right; it's a secret! Arthur has always wanted to keep the magical powers of the seahorse a mystery, but he also said that if anyone figured out what it does during their game play, we would confirm it.

I worked on every version of Fish Tycoon (desktop and mobile), and I can assure you that the seahorse does have a positive effect in the game. I could point to the exact location in the code where the effect is programmed. However, as borne out by the fact that nobody here has yet discovered the secret, the effect is not glaringly obvious, especially later in the game when there are a lot of other "magical things" working together.

That's all I'll say, since, well, it's a secret!